Two abundantly documented changes that have been observed in senescent cells from a wide variety of sources are (a) the accumulation in the senescent cells of altered proteins that lack enzymic activity, are heat labile, or are abnormally sensitive to attack by proteases, and (b) an enormous increase in the senescent cells of the volume occupied by lysosomes. Proteins that have been experimentally altered by the incorporation of certain amino acid analogs tend to be degraded very rapidly. There is some evidence for a deficiency of the degradative system responsible for the degradation of altered proteins in senescent cells. The possibility that defective protein degradation is responsible for the accumulation of altered proteins in senescent cells will be explored in two ways in human diploid cells in culture. A number of techniques will be used to detect the presence in senescent cells of proteins that escape normal protein turnover or turn over at a very low rate. And the degradation of experimentally altered proteins will be studied in detail. Proteins that escape degradation, whether naturally occurring or experimentally induced, will be characterized with respect to their intracellular location and chemical and physical properties. The ability of the lysosomes of senescent cells to maintain concentration gradients of hydrogen and potassium ions across their membranes will be determined using weakly basic substances that normally accumulate in lysosomes and ionophores that normally cause lysosomal swelling.